The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable receiving material, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common. One such plate is available from Eastman Kodak Company as the KODAK Direct Image Thermal Printing Plate. It includes an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste. Silicone rubbers, [such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)] have long been recognized as preferred waterless-ink repelling materials. The criteria for waterless lithography and the ink repelling properties of poly(siloxanes) have been extensively reviewed in the TAGA Proceedings 1975 pages 120, 177 and 195 and 1976 page 174. In addition to low surface energy, it was concluded that the ability to swell in long-chain alkane ink solvents (i.e., its "oleophilic" nature) accounts for silicone's superior ink-releasing characteristics. An important consideration is that siloxane polymers do repel ink.
In the lithographic art, materials that release or repel oil-based inks are usually referred to as having "oleophobic" character. Herein, ink repelling materials are defined as "melanophobic" and, conversely, the term "melanophilic" is used to describe ink "loving" or accepting materials.
The basic method of preparing a waterless printing plate involves the imagewise removal of silicone to expose an underlying ink accepting surface. For example, U.S. Pat. No. 3,677,178 (Gipe) discloses a waterless lithographic offset printing plate having a flexible substrate overcoated with a diazo layer that was in turn overcoated with silicone rubber. The plate was exposed to actinic radiation through a mask, initiating a reaction in the diazo layer that rendered the exposed areas insoluble. Development was accomplished by swabbing with a cotton pad containing water and a wetting agent to remove the unexposed coating areas.
It was recognized thereafter that a lithographic printing plate could be created containing an IR absorbing layer. Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose) And an optional cross-linkable resin. Such plates were exposed to focused near IR radiation with a Nd.sup.++ YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. The plate was developed by applying riaptha solvent to remove debris from the exposed image areas. Optionally, the unexposed areas could be cross-linked to improve adhesion of the background silicone layer. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. CO.sub.2 lasers are described for ablation of silicone layers by Nechiporenki & Markova, PrePrint 15th International IARIGAI Conference, June 1979, Lillehammer, Norway, Pira Abstract 02-79-02834.
More recently, WO 94/18005 discloses the use of dry cotton pads or non-solvent wiping to develop dry planographic plates after laser imaging.
Direct imaging on press or in a platesetter is also well known. In this case, the printing plates have layered structures wherein the layers having different affinities for ink and printing liquids are exposed to ablative absorption on press to create a printable lithographic surface. See, for example, U.S. Pat. No. 4,718,340 (Love III), WO 92/07716 (Landsman), U.S. Pat. No. 5,379,698 (Nowak et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,351,617 (Williams) and U.S. Pat. No. 5,353,705 (Lewis et al). In using these techniques, removal of the silicone rubber after exposure requires a development step that includes wiping.
Such printing plates typically have a layer or substrate that is melanophilic, and a layer or substrate that is melanophobic. The need to crosslink across the melanophilic, melanophobic interface in a printing plate has been recognized in the art as, for example, in EP-A 0 764522 and EP-A 0 763780. Crosslinking via thermally stable bonds results in relatively strong layers but makes thermal ablation difficult. Silicone rubbers are particularly difficult to ablate. Silicone debris c(lings to the support and to background areas and must be physically wiped away. Wiping has several drawbacks including the difficulty of reproducible removing all stray material with automated cleaning stations, and plate sensitivity to scratching from wiping.
A process for preparing dry planographic plates using copolymers of siloxane and crystallized thermoplastic blocks is known. U.S. Pat. No. 4,096,294 (Pacansky) describes the transfer of toner particles to the ink repellent receiver surface comprised of the siloxane and thermoplastic block copolymer. The thermoplastic phase is heated to improve the adhesion of the ink receptive toner particles to the receiver. Alkoxide crosslinking of organopolysiloxanes has also been disclosed for electrophotographic systems as, for example, in EP-B 0 028137. These methods suffer from the complexities of electrophotographic toner based systems. The structures do not contain photothermal conversion materials and therefore are unsuitable as direct thermal imaging plates.
It is well known to those skilled in the art that good adhesion is required of the layers that, in sum, comprise a printing plate. This is especially true for waterless plates due to the predominate use of silicone polymers in the ink repellent layer, owing to the low energy surface. While all current commercial waterless plates utilize a silicone polymer as the topmost layer, plates with an inverted structure have been generally described. This structure has the added challenge of preparing ink-accepting layers from low surface energy materials.
In U.S. Pat. No. 3,632,375 (Gipe), an uncured silicone rubber was dusted with a dry powder and subsequently cured and coated with a hydrophilic layer. Similarly, U.S. Pat. No. 3,949,142 (Doggett) teaches the coating of a photosensitive layer over an uncured silicone and curing the layers together. U.S. Pat. No. 4,086,093 (Ezumi et al) describes coating over a sparsely crosslinked silicone layer followed by a curing step. In U.S. Pat. No. 5,017,457 (Herrmann et al) a silicic acid intermediate layer and a diazo photosensitive resin were applied over an uncured silicone rubber layer. Similar approaches have been described in U.S. Pat. No. 3,728,123 (Gipe), DE 2,039,901, DE 2,361,815, DE 2,449,172 and U.S. Pat. No. 4,225,663 (Ball). These methods all require additional processing steps after imaging to produce the imaged printing plate.
Thermally sensitive printing plates that require no wet processing are needed in the industry. U.S. Ser. No. 08/816,287, now abandoned, "processless" direct write printing plates that can be imaged using lasers. Thus, these printing plates meet the needs of the art. However, there is a need for printing plates that image cleanly at relatively low exposure and exhibit good ink discrimination with little(e wear on press.